Re: [PATCH] hpfs: make check=none mount option excludable

[Mikulas Patocka <mikulas@twibright.com>]

On Tue, 14 Oct 2025, Tetsuo Handa wrote:

> syzbot is reporting use-after-free read problem when a crafted HPFS image
> was mounted with "check=none" option.
> 
> The "check=none" option is intended for only users who want maximum speed
> and use the filesystem only on trusted input. But fuzzers are for using
> the filesystem on untrusted input.
> 
> Mikulas Patocka (the HPFS maintainer) thinks that there is no need to add
> some middle ground where "check=none" would check some structures and won't
> check others. Therefore, to make sure that fuzzers and careful users do not
> by error specify "check=none" at runtime, make "check=none" being
> excludable at build time.

Hi

Would it be possible to change syzbot to not use the "check=none" option?

Mikulas

> Reported-by: syzbot+fa88eb476e42878f2844@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=fa88eb476e42878f2844
> Link: https://lkml.kernel.org/r/9ca81125-1c7b-ddaf-09ea-638bc5712632@redhat.com
> Tested-by: syzbot+fa88eb476e42878f2844@syzkaller.appspotmail.com
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  fs/hpfs/Kconfig | 11 +++++++++++
>  fs/hpfs/super.c |  2 ++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/fs/hpfs/Kconfig b/fs/hpfs/Kconfig
> index ac1e9318e65a..d3dfbe76be8a 100644
> --- a/fs/hpfs/Kconfig
> +++ b/fs/hpfs/Kconfig
> @@ -15,3 +15,14 @@ config HPFS_FS
>  
>  	  To compile this file system support as a module, choose M here: the
>  	  module will be called hpfs.  If unsure, say N.
> +
> +config HPFS_FS_ALLOW_NO_ERROR_CHECK_MODE
> +	bool "Allow no-error-check mode for maximum speed"
> +	depends on HPFS_FS
> +	default n
> +	help
> +	  This option enables check=none mount option. If check=none is
> +	  specified, users can expect maximum speed at the cost of minimum
> +	  robustness. Sane users should not specify check=none option, for e.g.
> +	  use-after-free bug will happen when the filesystem is corrupted or
> +	  crafted.
> diff --git a/fs/hpfs/super.c b/fs/hpfs/super.c
> index 8ab85e7ac91e..656b1ae01812 100644
> --- a/fs/hpfs/super.c
> +++ b/fs/hpfs/super.c
> @@ -285,7 +285,9 @@ static const struct constant_table hpfs_param_case[] = {
>  };
>  
>  static const struct constant_table hpfs_param_check[] = {
> +#ifdef CONFIG_HPFS_FS_ALLOW_NO_ERROR_CHECK_MODE
>  	{"none",	0},
> +#endif
>  	{"normal",	1},
>  	{"strict",	2},
>  	{}
> -- 
> 2.47.3
> 
> 
>